Lyndon N. Smith

Object surface recovery using a multi-light photometric stereo technique for non-Lambertian surfaces subject to shadows and specularities

This paper presents a new multi-light source photometric stereo system for reconstructing images of various characteristics of non-Lambertian rough surfaces with widely varying texture and specularity. Compared to the traditional three-light photometric stereo method, extra lights are employed using a hierarchical selection strategy to eliminate the effects of shadows and specularities, and to make the system more robust. We also show that six lights is the minimum needed in order to apply photometric stereo to the entire visible surface of any convex object. Experiments on synthetic and real scenes demonstrate that the proposed method can extract surface reflectance and orientation effectively, even in the presence of strong shadows and highlights. Hence, the method offers advantages in the recovery of dichromatic surfaces possessing rough texture or deeply relieved topographic features, with applications in reverse engineering and industrial surface inspection. Experimental results are presented in the paper.

3D face reconstructions from photometric stereo using near infrared and visible light

This paper seeks to advance the state-of-the-art in 3D face capture and processing via novel Photometric Stereo (PS) hardware and algorithms. The first contribution is a new high-speed 3D data capture system, which is capable of acquiring four raw images in approximately 20ms. The results presented in this paper demonstrate the feasibility of deploying the device in commercial settings. We show how the device can operate with either visible light or near infrared (NIR) light. The NIR light sources offer the advantages of being less intrusive and more covert than most existing face recognition methods allow. Furthermore, our experiments show that the accuracy of the reconstructions is also better using NIR light. The paper also presents a modified four-source PS algorithm which enhances the surface normal estimates by assigning a likelihood measure for each pixel being in a shadowed region. This likelihood measure is determined by the discrepancies between measured pixel brightnesses and expected values. Where the likelihood of shadow is high, then one light source is omitted from the computation for that pixel, otherwise a weighted combination of pixels is used to determine the surface normal. This means that the precise shadow boundary is not required by our method. The results section of the paper provides a detailed analysis of the methods presented and a comparison to ground truth. We also analyse the reflectance properties of a small number of skin samples to test the validity of the Lambertian model and point towards potential improvements to our method using the Oren-Nayar model.

Reflectance of human skin using colour photometric stereo: with particular application to pigmented lesion analysis

Background/purpose: The optical appearance of human skin is highly dependent on the interaction between the illumination (type and position), observer position and the skin surface structure. Different currently available photographic techniques record different aspects of this appearance, each providing its own incomplete description. This limits their usefulness, especially for pigmented skin lesion diagnosis. In this paper a new, easy to use, low‐cost photographic method is described,which aims to generate an efficiently encoded yet reasonably complete representation of skin appearance.

A mathematical morphology approach to image based 3D particle shape analysis

Angularity is a critically important property in terms of the performance of natural particulate materials. It is also one of the most difficult to measure objectively using traditional methods. Here we present an innovative and efficient approach to the determination of particle angularity using image analysis. The direct use of three-dimensional data offers a more robust solution than the two-dimensional methods proposed previously. The algorithm is based on the application of mathematical morphological techniques to range imagery, and effectively simulates the natural wear processes by which rock particles become rounded. The analysis of simulated volume loss is used to provide a valuable measure of angularity that is geometrically commensurate with the traditional definitions. Experimental data obtained using real particle samples are presented and results correlated with existing methods in order to demonstrate the validity of the new approach. The implementation of technologies such as these has the potential to offer significant process optimisation and environmental benefits to the producers of aggregates and their composites. The technique is theoretically extendable to the quantification of surface texture.

Face Recognition and Verification Using Photometric Stereo: The Photoface Database and a Comprehensive Evaluation

This paper presents a new database suitable for both 2-D and 3-D face recognition based on photometric stereo (PS): the Photoface database. The database was collected using a custom-made four-source PS device designed to enable data capture with minimal interaction necessary from the subjects. The device, which automatically detects the presence of a subject using ultrasound, was placed at the entrance to a busy workplace and captured 1839 sessions of face images with natural pose and expression. This meant that the acquired data is more realistic for everyday use than existing databases and is, therefore, an invaluable test bed for state-of-the-art recognition algorithms. The paper also presents experiments of various face recognition and verification algorithms using the albedo, surface normals, and recovered depth maps. Finally, we have conducted experiments in order to demonstrate how different methods in the pipeline of PS (i.e., normal field computation and depth map reconstruction) affect recognition and verification performance. These experiments help to 1) demonstrate the usefulness of PS, and our device in particular, for minimal-interaction face recognition, and 2) highlight the optimal reconstruction and recognition algorithms for use with natural-expression PS data. The database can be downloaded from http://www.uwe.ac.uk/research/Photoface.

Dynamic photometric stereo-a new technique for moving surface analysis

This paper describes a novel approach for two- and three-dimensional surface data capture from moving surfaces based upon an evolution of the existing photometric stereo (PS) technique. Limitations in current methods are described, together with the potential benefits of applying PS and the particular need for a new dynamic form of the method. Important issues relating to conventional idealised PS model assumptions are considered in the context of realising useful practical application, including in particular the modelling of real illuminates. Several possible techniques for achieving dynamic PS are considered and a new technique termed narrow infrared photometric stereo (NIRPS) introduced. New potential application areas range from the continuous inspection of fast moving surfaces typically encountered in numerous industrial processes to three-dimensional surface topographic texture acquisition in the field using portable hand-held technology. A selection of experimental results is presented in the paper.

Dynamic photometric stereo for on line quality control of ceramic tiles

The rapid and automated detection of manufacturing flaws is becoming increasingly important in order to maintain competitive advantage in many production environments. In the case of natural and ornamental materials, the presence of both surface colouration and surface topography is often such that manual inspection, along with many conventional imaging techniques, fails to isolate physical or structural defects in the presence of complex and random patterns. In this paper the concepts of photometric stereo are adapted and extended for application in manufacturing environments. A case study on the high speed inspection of ceramic tiles is presented for the analysis of surfaces at production line rates of up to 30 m/min. This new technique, for the first time, demonstrates a genuine and commercially attractive potential for the practical automated quality control of complex surfaces. A commercial system, based on this research, is currently being developed.

Overview of passive and active vision techniques for hand-held 3D data acquistion

The digitization of the 3D shape of real objects is a rapidly expanding discipline, with a wide variety of applications, including shape acquisition, inspection, reverse engineering, gauging and robot navigation. Developments in computer product design techniques, automated production, and the need for close manufacturing tolerances will be facts of life for the foreseeable future. A growing need exists for fast, accurate, portable, non-contact 3D sensors. However, in order for 3D scanning to become more commonplace, new methods are needed for easily, quickly and robustly acquiring accurate full geometric models of complex objects using low cost technology. In this paper, a brief survey is presented of current scanning technologies available for acquiring range data. An overview is provided of current 3D-shape acquisition using both active and passive vision techniques. Each technique is explained in terms of its configuration, principle of operation, and the inherent advantages and limitations. A separate section then focuses on the implications of scannerless scanning for hand held technology, after which the current status of 3D acquisition using handheld technology, together with related issues concerning implementation, is considered more fully. Finally, conclusions for further developments in handheld devices are discussed. This paper may be of particular benefit to new comers in this field.

The Photoface database

In this paper we present a new database suitable for both 2D and 3D face recognition based on photometric stereo, the so-called Photoface database. The Photoface database was collected using a custom-made four-source photometric stereo device that could be easily deployed in commercial settings. Unlike other publicly available databases the level of cooperation between subjects and the capture mechanism was minimal. The proposed device may also be used, to capture 3D expressive faces. Apart from the description of the device and the Photoface database, we present experiments from baseline face recognition and verification algorithms using albedo, normals and the recovered depth maps. Finally, we have conducted experiments in order to demonstrate how different methods in the pipeline of photometric stereo (i.e. normal field computation and depth map reconstruction methods) affect recognition/verification performance.

Examining the uncertainty of the recovered surface normal in three light photometric stereo

Although the three light photometric stereo technique has been used in many applications, there is little published work concerned with characterizing the uncertainty of these systems due to the involvement of a number of complicating factors. This paper presents a methodology used to analyze the uncertainty of the recovered unit surface normal with respect to irradiance variance. Illumination configurations and the values of the composite albedo are found to directly affect the stability of the photometric stereo technique. An orthogonally distributed illumination arrangement is proven to be the theoretically optimal configuration. Further practical considerations are also identified. The derived general uncertainty expression can be easily employed to optimize the location of the light sources. Hence, the work is of significance for the development of practical industrial applications of photometric stereo, including metrology, reverse engineering and various surface inspection tasks.